The field of the invention relates generally to a probe for imaging or for effecting change in state at the atomic and near atomic level. In particular, the field of the invention relates to a monolithic probe for scanning probe microscopy applications, such as atomic force microscopy, as well as field emission tips, or the like, for effecting a change in state at the atomic level, and to a method for making such a probe in a reproducible, high volume manner.
Scanning Probe Microscopy (SPM) is used to describe many different methods which use a force interaction between a nanometer size point on a tip and a sample surface to physically sense forces associated with a surface and to image that surface with subnanometer resolution. This includes atomic force microscopy (AFM), magnetic force microscopy (MFM), scanning tunneling microscopy (STM), scanning thermal microscopy (SThM), and like methods. A related aspect of SPM also uses a nanometer size tip such as an AFM or like tip to emit electrons for effecting a switching phenomenon, such as a change of state, change of phase, or localized change in resistivity or optical property at or near the surface of a material at the atomic scale. This has been used to demonstrate that it is now possible to write information on a surface using single atoms. In one application, this has been done by positioning individual atoms adsorbed on a single crystal metal surface by means of a scanning tunneling microscope tip assembly.
The most critical part of any SPM is the probe. A probe typically comprises a mounting block, cantilever, and a tip assembly depending from the cantilever. The tip assembly further comprises a high aspect ratio column and a tip having substantially atomic sharpness disposed on the distal end of the column. The tip of the tip assembly follows the surface to be imaged, and in most instances, moves up or down due to the tip interacting in some manner with atoms on the surface being imaged. In many SPM applications, the deflection of the cantilever is used to measure the interaction between the tip and the sample being scanned. The deflection of the tip is translated by techniques which are well known into an image of the surface. Other SPM applications utilize changes in resonant frequency or electrical current to measure interaction between the tip and the sample.
Various techniques of scanning probe microscopy exist. Each technique has particular requirements that the probe must meet to achieve the desired objective of creating an accurate image of a target surface, or properties of that surface, with substantially atomic resolution.
AFM measures variations in the repulsive forces between the atoms on the surface to be imaged and the point of the tip of a tip assembly, which point is less than about 30 nanometers in diameter (i.e. it is "atomically sharp", meaning it is able to resolve images at an atomic level). The principle of atomic force microscopy (AFM) operation can be applied to measure a variety of forces and image those forces, including ionic repulsion, van der Waals forces, capillary, electrostatic, magnetic and frictional forces. A major impact of AFM can be expected to be made in many different research fields such as biology, electronics, and condensed matter physics. For example, magnetic force microscopy, an AFM technique, has now become an established experimental technique for the study of surface magnetic properties.
One technique for AFM uses a tip assembly in a non-contact mode for imaging a sample surface. In the non-contact mode, the tip is maintained at a distance of approximately 50-150 .ANG. above the surface. This is necessary in situations in which tip contact may alter the sample surface. The interaction force between the tip and the sample surface being imaged causes a measurable change in the cantilever status, such as deflection or shift in resonant frequency. Several techniques have been developed to detect the interaction force between the tip and sample.
The most straightforward technique detects the force by measuring the static deflection of the cantilever, such as by an optical interferometer or by an integrated strain gauge such as a piezoresister (see for example Minne et. al., APL 67, Dec. 25, 1995). Significantly greater sensitivity to extremely small forces such as van der Waals forces, or the like, can be obtained by exciting the cantilever to mechanical resonance and measuring changes in the resonant frequency. In this method, a change in the resonant frequency of the cantilever occurs due to atomic force interactions between the tip and the sample as the tip approaches the sample. In another technique, a change in the resonant frequency of the cantilever is detected by keeping the excitation frequency constant and measuring a change in the vibration amplitude of the cantilever using a sensitive laser heterodyne probe. Thus, the resonant frequency of the cantilever is one critical parameter that must be carefully controlled in order to consistently image surfaces.
An atomic force microscope also can be used as a probe for tracking ultra high speed pulses in microelectronic devices. In this application, the AFM tip stays about 2 .ANG. above the sample surface. The tip is kept at that position by a gentle downward pressure exerted by the cantilever and a quantum mechanical repulsive force that prevents the tip from coming closer to the target surface. A stream of voltage pulses are applied to the tip and to the device under test. The stream of pulses to the tip enable the AFM to be used as an extremely fast mixer or sampler. Sampling is necessary because the cantilever cannot vibrate fast enough to keep up with the voltage pulses. This enables a researcher to take stroboscopic like images of waveforms. Very short pulses to the AFM tip can freeze the action of a high speed waveform. The application of signal mixing utilizes pulses up to 20 GHz and sampling with 100 picosecond time resolution. This enables a researcher to create logic maps of ultra high speed nano scale circuits by scanning their topographies and voltage characteristics at the same time.
In the magnetic force microscope (MFM), a magnetic tip is mounted on a flexible cantilever and is used to image magnetic field patterns. The magnetic dipole of the tip interacts with the stray magnetic fields from a sample surface. Stray magnetic fields exert a force on the magnetized tip. The gradient of stray magnetic fields alters the resonant frequency of the cantilever. The change in resonant frequency is used to provide information regarding the sample surface.
The shape and magnetic properties of the tip used in MFM are critical to obtaining a quantative measure of the sample surface of magnetic properties. See for example Proksch et al., APL 66, May 8, 1995, p. 2582.
In STM applications, the tip is held at a constant tunneling current by maintaining a constant vertical height from the image to be scanned. A constant vertical height is maintained by moving the tip assembly up or down on the Z-axis. Movement of the tip is therefore achieved by external control and not by a force directly acting between the tip and the surface.
A scanning thermal (SThM) probe can be used to profile atomic features on both insulating and conducting crystals, or the like. The scanning thermal probe comprises a thermal sensor, such a thermocouple, disposed at the apex of a probe tip. When a constant current is passed through the thermocouple junction, the thermocouple heats up to an equilibrium temperature above the ambient value. If the tip now approaches a sample surface (an insulator, conductor or even a liquid) it cools down due to heat transfer from the tip to the sample. The tip temperature which is detected by the thermocouple then can used to control tip-spacing in much the same way as a tunneling current is used in a STM as the tip is scanned across the surface. When the tip is vibrated by a few tens of angstroms (.ANG.) in the vertical direction, the ac change in the thermoelectric voltage is used as a monitor of the tip-sample spacing. This renders the system immune to ambient temperature variations caused by room temperature fluctuations and air currents in the vicinity of the probe tip.
A SThM probe can also be used to measure surface feature temperatures or thermal conductivities with high spatial resolution. See for example Lai et al., "Thermal Detection of Device Failure by Atomic Force Microscopy," IEEE Electron Device Lett., 16 (1995), pp. 312-315. In this application, the scanning probe is thermally activated and sensed to obtain both a topographic and thermal image of the surface.
Another application relating to SPM uses a probe tip having a nanometer size apex to emit electrons from the apex to the sample surface. The electron emission is used to effect a switching phenomenon in the sample surface at the atomic scale. The switching phenomenon is typically a change of state, change of phase, localized change in resistivity or optical property, or the like.
Yet another SPM application uses the electrical interaction between AFM tip and sample surface to produce a localized enhanced oxidation of the sample (Dagata, Science 270, Dec. 8, 1995). This technique can be used to fabricate electronic devices with dimensions of 10 nm or less.
The foregoing SPM methods mandate a number of requirements that probes must exhibit to successfully implement SPM. The tip assemblies must have the following associated characteristics.
1) The mounting block must be characterized by a geometry which is suited to the particular use of the probe. The mounting block must fit into the SPM, or like instrument, as well as be of sufficient size such that it can be manipulated by the operator of the instrument. This is critical since only the mounting block is large enough to be physically manipulated by the operator of the instrument. Furthermore, the mounting block must be characterized by sufficient rigidity to provide a stable and rigid support for the cantilever. The relationship of the cantilever to the mounting block must be consistent. That is, the cantilever must always be in the same place on the mounting block and extend outwardly for a predetermined distance for each probe made for a specific application. The mounting block must be supported by a larger structure, such as a silicon wafer, from which it is processed in such a way that the mounting block can undergo necessary processing and yet easily be released from the wafer without damage upon completion of processing. Further, the mounting block must be of sufficient dimensions to allow ready attachment to the SPM device by an operator.
2) The cantilever must have suitable dimensions and mechanical properties to control the cantilever spring constant and resonant frequency. Once the desired tip geometry is determined, the dimensions and material from which the cantilever are made determine the resonant frequency, spring constant, and Q value of the cantilever. Detection of the shift in resonant frequency of the cantilever is a powerful method for measuring narrow spacings and thus imaging a sample surface. The most sensitive force microscopes use resonance enhancement of a vibrating cantilever to detect forces as small as 3.times.10.sup.-13 N. With resonance enhancement, either a shift in the resonant frequency or a decrease in the vibration amplitude can be used for proximity detection. Monitoring the vibration amplitude is equivalent to measuring the Q of the vibrating lever as it interacts with the surface.
The cantilevers in scanning force microscopy applications need a small spring constant to achieve high sensitivity, but also require a high resonant frequency to achieve reasonable scanning speed, and to render the cantilever insensitive to acoustic noise and external vibrations. The limited ability of conventional SPM probes to operate at high scanning speeds is a well known problem (Manalis et al., APL 68, Feb. 5, 1996). In order to achieve low spring constant and high resonant frequency simultaneously, the cantilever must have low mass, as shown by the following relationship of resonant frequency .nu. with spring constant k and cantilever mass m: ##EQU1## A low mass can be achieved by making the cantilevers physically small. Typical cantilevers are 100-400 microns (.mu.m) in length, 10-50 .mu.m in width, and have a thickness in a range of from 0.1-10 .mu.m. Thus, the size of the cantilever and the tip must be carefully controlled to assure that the cantilever has the correct resonant frequency, spring constant, and Q value for use in a particular SPM method.
The cantilever must further have dimensions appropriate to the measurement being performed. For example, if a laser interferometer is used to measure cantilever displacement, the cantilever must be of sufficient width such that the reflected laser light can be accurately determined.
3) The tip assembly must be suited to the particular use. Image formation at the subnanometer level is not a linear process, and the size and shape of the tip are critical. Accurate measurement of a sample surface is not possible unless the dimensions of the probe are known and are consistent from one tip assembly to the next. If the size and shape of a probe are not accurately known and the size and shape vary from probe to probe, distortions in measurement may be induced. Such distortions must then be removed by complex compensation circuitry or software.
To image high topology surfaces, the tip assembly must comprise a high aspect ratio column depending from the cantilever. The distal end of the column culminates in a tip which is preferably characterized by a high cone angle. The column and tip must be characterized by high torsional rigidity. The column preferably tapers to a minimal diameter tip. This advantageously enables the tip to enter high topology surfaces for accurate imaging. Thus, the tip should be characterized by a high cone angle and a diameter of less than 30 nanometers.
The size of the tip must be similar to or smaller than that of the object to be studied. To achieve atomic resolution, the tip must end with a small cluster of atoms. To scan a micron deep hole, the tip must not exceed the diameter of the hole for a full micron back from the apex. In addition, an extremely narrow tip with a high cone angle may be necessary to measure samples with steep or reentrant topography. This must be achieved without comprising the stiffness or stability of the tip. Reproducibly making such high aspect ratio probes is one of the most difficult tasks in conventional scanning force microscopy.
As shown in FIGS. 1A-C, distortions in measurement are caused by a typical probe-sample interaction. In FIG. 1A, a point 102 of a probe 100 may be too blunt or have a diameter too large to reach the bottom of a trench in an object 106 to be scanned. The true depth of the trench cannot be extracted from the scan 104. (Drawings 1A-C are adapted from Journal of Applied Physics 74(9) Nov. 1, 1993, pp. R83-R109.)
A serious problem in conventional scanning force microscopy arises when a surface to be imaged has regions with steep slopes. On lithographically patterned surfaces, the problem is especially acute. Deep, narrow trenches and holes with undercut sidewalls are common. The ability of a scanning force microscope probe to accurately image surface topography depends strongly on the size and shape of the probe.
FIG. 1B shows the desirability of a high cone angle for a scanning force microscope probe. The angle .alpha. is greater than .beta. so that the tip with such a high cone angle .alpha. faithfully follows the left side of the trench. Such a probe is limited only in its ability to image the slope of the right side of the trench.
FIG. 1C also shows a probe with a high cone angle as in FIG. 1B but having a tip point with a large radius of curvature. In FIG. 1C, the scan line 104 produced by a blunt probe 102 encountering a sudden step creates a distortion in the image between 1 and 2. The region between 1 and 2 may be referred to as a "dead zone" because it cannot be reached by this probe. As shown in FIG. 1D and in the previous figures, the distortions caused by the size and shape of the tip or point can result in major inaccuracies in the imaging of a surface 112 and are a major problem in conventional scanning force microscopy.
Accordingly, in view of the foregoing requirements, what is needed is the ability to independently control the dimensions of the tip, its supporting column and cantilever.
4) The probe must also have a rigidity that is sufficient to maintain a constant dimensional relationship between the mounting block, cantilever and tip assembly. A lack of stiffness in the probe induces strong distortions in the measurement signal. External vibrations easily superimpose themselves on the cantilever vibrations used in imaging the surface when the tip assembly has insufficient stiffness or rigidity. Also, the probe itself may deform as it is scanned, resulting in noise and false images. A rigid probe and tip assembly are therefore an essential requirement for consistent imaging.
5) The probe must have consistent and reproducible dimensions. In order to provide consistently accurate images of a surface, a probe used for, e.g. AFM, must have the same dimensions as another probe made weeks or months later. The probe must have substantially identical mounting blocks, cantilever structures, resonant frequencies, and tip size and geometry. Few or no imaging instrument adjustments should be necessary to compensate for probe variations in geometry.
Conventional methods of manufacturing a cantilever and associated sensor tip are inadequate to reproducibly form tips capable of operating at the high frequency which may be required for adequate imaging. In order to achieve the objective of atomic resolution, the tip size must be comparable to atomic dimensions. Because the resonant frequency of the cantilever beam also plays a critical role in imaging as previously explained, the probe must not only be reproducible but must be made with constant dimensional parameters in order to control the mechanical resonant properties of the cantilever beam. Presently, tips and cantilever beams produced by typical wet chemical etching techniques are not adequately reproducible. Conventional scanning probe fabrication processes when applied to making a plurality of tips across a single wafer, may result in tips which are of highly nonuniform shape (U.S. Pat. No. 5,201,992, column 5, lines 53-54). Thus, what is needed is a method for batch fabrication of an array of AFM probes which are highly uniform and which are capable of being fabricated with controlled geometry. Also, it is presently not possible to achieve fabrication of an array of SPM tip assemblies with any degree of reproducibility. Thus, probes presently are not interchangeable with a sufficient degree of predictability. The dimensions of the plurality of SPM probes in an array cannot be maintained with uniformity across a single wafer.
6) The method of manufacturing must achieve a high volume production of probes. Due to the tremendous increase in diverse SPM and AFM applications, there is a need for easily reproducible, mass produced probes. During routine use, tips easily can become contaminated or damaged and need to be replaced.
Thus, what is needed is a process for reproducibly fabricating rigid probes of increasing geometric complexity such as tips for biomolecular imaging, CD measurement, or for storage applications. Such a process should be capable of reproducibly fabricating a variety of geometries depending upon the particular application (e.g. for AFM, MFM, STM, or other applications). The tips should be characterized by substantially atomic sharpness which easily can be reproduced and fabricated in large quantities with a high degree of precision.
What is also needed is a process for reproducibly manufacturing a scanning force microscope tip with a high cone angle, a high degree of stiffness, and with predictable reproducibility and high yield. This would be a great advantage over conventional methods of producing scanning force microscope tips. Such a tip assembly would be capable of producing accurate images of surfaces having a slope less than that of the cone angle of the tip.
Conventional scanning probe microscopy tip assemblies have critical problems concerning lack of reproducibility and inability to make rigid tip assemblies to tight manufacturing tolerances. The development and improvement of scanning probe microscopy techniques is strongly dependent upon the development of appropriate probes. Suitable tips must be capable of being fabricated easily and reproducibly in large quantities with substantially atomic sharpness. What is needed is a method for making probes by mass production techniques. What is also needed is a method for making a plurality of probes having consistent or substantially unformed properties from a single wafer. This is necessary because sensor tip damage or contamination for an atomically sharp tip easily occurs in SPM experiments due to the extremely small separation between the sensor and the sample surface.
It is also required that the fabrication process allows for largely independent control over mounting block, cantilever, and tip assembly dimensions such that the probe can be tailored to meet the requirements of specific SPM applications. For example, in one application the AFM is mounted in a scanning electron microscope (SEM) such that the tip and the area being scanned by the tip can be imaged by the SEM while the AFM is in operation. This application requires that the tip assembly have a height greater than one-half the width of the cantilever such that when the probe is mounted above the sample, the distal end of the tip is visible to the scanning electron beam. To facilitate this application it is possible to create a cantilever which has a reduced width, e.g. is triangular in plan view, in the vicinity of the tip assembly.
Up to now, basically several techniques have been used for the production of tips and cantilevers. In one technique, a thin wire or piece of metallic foil is bent and etched electrochemically. As known from the production of STM tips, a radius of curvature of less than 1,000 .ANG. can be prepared by this method. However, tips formed by this method are difficult to prepare and are not easily reproduced at critically small dimensions. This method is also not easily adapted to making large numbers of tips concurrently and to high accuracy. Therefore, this method has been largely abandoned for all but a few applications.
Another method for cantilever preparation involves producing SiO.sub.2 cantilevers which are rectangular or triangular in shape by standard etching techniques of an oxidized Si wafer. Standard photo masks are used to define the shape of the cantilevers, so that the geometry is known and spring constants can be calculated. A probing tip is provided by tilting a corner of the cantilever toward the sample. The sharpness of such tips is not well controlled and as a consequence, multi-tip effects can become a severe problem.
Some progress has been made in the use of Si.sub.3 N.sub.4 instead of SiO.sub.2 as a cantilever material. See U.S. Pat. No. 5,066,358 as an example. Si.sub.3 N.sub.4 cantilevers are less fragile, and the thickness can be reduced from approximately 1.5 to 0.3 .mu.m. However, such cantilevers have low stiffness and low resonant frequency.
One conventional method of making a tip involves etching a pyramidal pit into a silicon wafer. See U.S. Pat. No. 5,116,462 as an example. Afterwards, a film of Si.sub.3 N.sub.4 is deposited which follows the contours of the silicon. Si.sub.3 N.sub.4 is also patterned into the shape of a cantilever. When the silicon is etched away from around the cantilever, the free standing cantilevers have pyramidal Si.sub.3 N.sub.4 tips which are a replica of the previous pyramidal mold formed in the silicon. Although silicon nitride (Si.sub.3 N.sub.4) tips may be fabricated with some degree of reproducibility, such cantilevers are limited in their resonant frequency and cannot be used for high frequency applications. Also, Si.sub.3 N.sub.4 tips have cone angles which are constrained to the angles formed by intersecting &lt;1:1:1&gt; planes. This property makes such tips unsuitable for metrology applications.
What is needed is a new application of silicon based manufacturing techniques to achieve silicon microstructures which are suitable for probes. To date, the manufacture of single crystal silicon tips has been characterized by the use of wet chemical etchants to form the tip and cantilever. Present AFM tips and cantilevers manufactured by isotropic and anisotropic wet chemical etching techniques (such as etching with aqueous potassium hydroxide, KOH) are characterized by inhomogeneity and lack constant reproducibility.
An approach to making a sharp silicon tip is set forth in Marcus et al., U.S. Pat. No. 5,201,992. This patent teaches the use of conventional wet-chemical etching to form a silicon post (protuberance) and oxidation to sharpen the post into a tip. This method has several disadvantages inherent to wet chemical etching techniques. First, in order to prevent the formation of blunt, rounded tips, the etching must be terminated before mask islands become detached from the silicon tapers forming under the mask islands. It is extremely difficult to stop wet-chemical etching by timing the etch rate with any degree of predictability. This process cannot fabricate tips with a high degree of reproducibility. There is little margin for error, and such wet chemical etches are inherently non-reproducible.
After etching the protuberances, U.S. Pat. No. 5,201,992 teaches sharpening the protuberances by oxidation of those structures and removing the surface oxide in concentrated hydrofluoric acid or the like. It is noted at column 5, lines 53-54 of U.S. Pat. No. 5,201,992 that the process is not reproducible for a plurality of structures, and the process results in a plurality of structures that are of highly nonuniform shape. This method has a severe disadvantage in that it is inherently unreproducible. The inability to control the etch rate has the further disadvantage that final geometry of the cantilever and tip cannot be determined with any degree of precision. Further, U.S. Pat. No. 5,201,992 does not teach how a tip could be integrated with a cantilever and mounting block to form a probe suitable for SPM applications.
Bayer et al., U.S. Pat. No. 5,051,379 teach a method for producing a micromechanical sensor for AFM/STM profilometry. The sensor comprises a cantilever beam with a tip at one end and a mounting block at the opposite end. However, the '379 patent fails to disclose or to suggest how a mounting block could be produced using the method described. In a manner similar to the above referenced '992 patent, the method disclosed in the '379 patent preferably utilizes a wet chemical etch, to undercut the mask material and form a silicon tip. This method again has the disadvantage that exquisite control over the etch is required to reproducibly form sharp tips. It is noted at column 6, lines 28-30, that control of the etch timing is critical and in-process monitoring through optical inspection is required. The '379 patent further acknowledges the deficiency of this method at column 4, line 64, where it is noted that ion milling may be required post tip formation to sharpen the tip. Thus, this process in practice lacks sufficient control to reliably form the desired tips.
In a second embodiment, the '379 patent discloses a single crystal structure fabricated by wet chemical etching. The tip is etched on higher order crystal planes using specific conditions for an anisotropic wet chemical etchant. Control of the final tip dimensions, especially tip height, using this technique is extremely difficult, and thus has all of the inherent problems of process control and lack of reproducibility mentioned above.
U.S. Pat. No. 5,282,924 shows an attempt to make a reproducible and uniform cantilever and tip. There, the cantilever beam with an integrated tip is anisotropically etched out of a single-layer silicon wafer. In the '924 method, the single-layer wafer is thinned from the bottom by wet-chemical etching to a thickness which corresponds to about the thickness of the cantilever beam plus twice the height of the tip, plus a residual wafer thickness which is consumed during thermal oxidation.
The '924 process has a similar disadvantage to that of the '379 patent in that the wet chemical etching process is extremely difficult to control. Independent factors such as location and shape of edges, amount and shape of undercut of the silicon surface protected by the mask, differences in etchant concentration, temperature variations and gradients, and degree of agitation make it virtually impossible to control the final geometry of the tip and cantilever, especially the height and thickness respectively, with any degree of reproducibility. This in turn adversely affects the reproducibility of factors critical to the performance of the tip assembly, such as resonant frequency, sensitivity, and suitability for metrology. Thus, the final product lacks the consistency needed to provide accurate and repeatable images.
The '924 patent has a further disadvantage in that the cantilever mask is created prior to the tip mask. The tip lithography is the most demanding because it is the smallest feature of the probe and its placement requires the greatest accuracy. When the cantilever mask is created prior to the tip mask, the tip lithography must be performed on a surface with topography created by the cantilever mask. A planar surface must be provided for the most accurate tip lithography and in '924 this is compromised by the existing cantilever mask.
Lastly, U.S. Pat. No. 5,282,924 fails to suggest how the tip and cantilever fabricated using the process described could be integrated with a mounting block to form a probe suitable for AFM applications. All known anisotropic etchants undercut convex corners in the etching of (100) silicon. The '924 patent fails to disclose how it is possible to achieve uniform rectangular structures without some form of compensation. Uniformity of dimensions could not be achieved using the timed back side etching process of the '924 patent without some form of compensation. The '924 patent further fails to disclose a means for supporting the fragile cantilever and tip structures during processing and handling.
In order to overcome the problems inherent in attempting to control the final geometry, specifically the cantilever thickness and tip height, by timing the etch rate, some conventional methods implant boron into silicon in order to provide an etch stop. When the etching solution encounters a very high concentration of boron in silicon, the etch rate will drop by a factor of approximately 100.
There are significant disadvantages to this method. Initially, it introduces another complex process step of attempting to implant boron at a desired concentration and depth. This is extremely difficult if one attempts to implant boron to the desired depth of the tip height and cantilever thickness, in excess of 4 microns. Typical implantations are limited to approximately 1 micron. Often, epitaxial growth of silicon is employed after the implantation to increase the thickness of the silicon layer, but this process is expensive, time consuming, and the quality of the epitaxial layer is compromised by the high degree of crystal damage in the implanted region. Damage introduced by the implantation can also disadvantageously effect the mechanical properties of the cantilever, including resonant frequency (see Pember et al., APL 66, Jan. 30, 1995, p. 577). Thus, conventional methods of boron implantation severely limit the final geometry and reproducibility of the cantilever and tip.
In an attempt to overcome some of the described problems inherent in controlling the rate and extent of etching solutions, U.S. Pat. No. 5,354,985 discloses a method for forming a cantilever beam and tip for a near field scanning optical microscope (NSOM) utilizing a silicon-on-insulator (SOI) wafer. An NSOM probe is unsuitable for scanning probe microscopy. The cantilever beam of the '985 patent must be sufficiently large to include a wave guide to carry light to the tip. In addition, the tip must be provided with an aperture for emitting photons. '985 is not concerned with supplying a tip of atomic sharpness. The NSOM probe does not require the tightly-controlled dimensions that are so critical to the performance of probes used in scanning probe microscopy.
As a result, the process of making NSOM probes disclosed in '985 differs substantially from a process suitable for making SPM probes. The insulator layer of the SOI wafer used in this process has little influence in determining the final tip and cantilever height.
Another conventional attempt to make accurately-dimensioned SPM probes suitable for AFM was disclosed by J. Itoh, Y. Tohma, S. Kanemaru, and K. Shimizu, Fabrication of an Ultrasharp and High-aspect-ratio Microprobe with a Silicon-on-iisulator Wafer for Scanning Force Microscopy, 13 J. VAC. Sci. TECHNOL. 2, 331-34 (Mar./Apr. 1995). This article discloses that a probe comprising a silicon tip and mounting block and a silica cantilever can be made using a SOI wafer. The authors note that a silica cantilever has a low Q value that is insufficient for certain applications, and addition of another material such as silicon after formation of the probe may be necessary to increase the Q value. Also, the tip is sharpened using a KOH etch on higher-order Si planes. Using KOH etch produces unpredictable variations in dimensions, especially in the height of the tip. As a result, the tips formed by this method cannot be made to highly accurate dimensions nor are they easily reproducible.
Thus, none of the foregoing conventional approaches presents a method for producing a probe that has all six required characteristics for SPM as described above. Accordingly, there is a need for a process which can fabricate a SPM probe including a mounting block, cantilever and integrally formed tip assembly with a high degree of reproducibility using batch fabrication techniques that are typically encountered in electronic component fabrication. What is also needed is a process does not rely upon fabrication parameters such as timing the etch rate of wet-chemical, anisotropic or isotropic, etching to form such critical components as the tip and cantilever.
There is also a need for a method for making a probe including a mounting block, cantilever and integrally formed tip in a reproducible manner with a high degree of control over the final geometry of the tip and cantilever in order to optimize the probe performance for specialized applications such as DNA sequencing, CD measurement, high scanning speeds, or measurement of high aspect ratio via holes in integrated circuits. For example, in biomolecular imaging it is desirable to have a much taller cantilever and a longer tip with a high modulus of elasticity. Using the conventional methods described above, it is presently not possible to control the geometry of the tip with a sufficient degree of reproducibility. Thus, the tip assemblies presently marketed for biomolecular imaging, for example, represent a select few from the many tip assemblies which are made but rejected as non-conforming to dimensional specifications. Such tips assemblies are therefore extremely expensive.
It also would be desirable to have independent control over cantilever and tip dimensions such that tall tips could be fabricated on appropriately shaped cantilevers to enable viewing of the tip, in for example a scanning electron microscope, during SPM operation.
It also would be desirable to fabricate a probe including a monolithic mounting block, cantilever and tip assembly with the ability to control the tip geometry to provide a tip that is characterized by substantially atomic sharpness and is ideally of uniform dimensions in order to measure a variety of forces at the atomic level, including ionic repulsion, van der Waals forces, capillary, electrostatic, magnetic, and frictional forces.
It also would be desirable to be able to fabricate with batch techniques and high reproducibility, a probe wherein the tip geometry is controllable to provide a small radius of curvature and a small cone angle (high aspect ratio) for achieving substantially atomic resolution.
It also would be desirable to fabricate a monolithic probe including a tip with well controlled geometry and substantially atomic sharpness which can be used to emit electrons to effect a change in static or change in material properties on a neon atomic scale.